f.c.c./f.c.c.LUCS的电子结构

在只计入最近邻间相互作用的紧束缚近似下,计算了由两种面心立方金属组成的多层超薄共格结构(LUCS)的电子能谱和态密度。 关键词：     

Relative stability of f.c.c. and b.c.c. structures for model systems at high temperatures

Summary Free-energy, entropy and volume differences between face-centered and body-centered cubic structures have been evaluated for model rare gas and alkali metal crystals by using the method of overlapping distributions. Stable phases are predicted in agreement with the behaviour of real materials in the regions of validity of classical mechanics and in agreement with the results of previous dynamical-simulation studies of crystal nucleation from the melt and of polymorphic transformations. The existence of a stable b.c.c. phase at high pressure and temperatures is predicted in this way for Lennard-Jones solids, while no high-pressure f.c.c. phase is expected for model Rb and Cs systems. We also show the possibility of making calculations of free-energy barriers to displacive crystalline transformations along a prescribed trajectory in configuration space. Work supported by the U.S. Department of Energy, Istituto per la Ricerca Scientifica e Tecnologica, Trento, and Gruppo Nazionale di Struttura della Materia del C.N.R., Italy.     

B.c.c.-f.c.c. allotropy, F-bands,and metallization in xenon and krypton

APW-Xα calculations of a f.c.c.-b.c.c. phase transition in Xe prior to metallization give P ? 950 kbar, contrary to the 820 kbar result of Hama and Matsui; their use of non-touching APW spheres (f.c.c.) may be the crucial difference. Hartree-Fock plus correlation calculations in Kr indicate that f-bands play no role in rare gas crystal metallization.     

B c decays

Theoretical predictions on decay properties of the simplest particle containing more than one heavy quark, the pseudoscalar mesonB _c ⁺ , are presented. Some channels that should present a distinctive signature are discussed in more detail.     

h.c.p.-f.c.c. phase transition in solid parahydrogen

The difference of free energies for the h.c.p. and f.c.c. parahydrogen is calculated as a function of molar volume. It is shown that the observed transition in solid parahydrogen¹ can be explained by differences in the structure of the cubic and quartic anharmonic terms.     

F.c.c. type-III antiferromagnetism in YbN

Neutron diffraction experiments performed for YbN in the temperature range from 7 mK to room temperature prove the existence of long-range f.c.c. antiferromagnetic ordering of type III belowT _N =(0.790±0.005) K. The low ordered magnetic moment of Yb at saturation, =(0.39±0.05)> _B , is presumably caused by Kondo hybridization. By means of inelastic neutron scattering the crystalfield level scheme was established to be ₆ – ₈(33 meV)– ₇(81 meV). The ₆ – ₈ transition was found to be split into two lines which may be due to a bound state between the crystal-field excitation and phonons.     

Search for the ρρ enhancement near 1700 MeV/c in the reaction at 8.3 GeV/c

Results are presented from a search for a ρ⁰ρ⁰ enhancement in antiproton annihilations. A ρ⁰ρ⁰ resonance was recently observed in radiative ψ decays, and its existence has been supported by the results of an antiproton experiment at 5.7 GeV/c. No indication of this ρ⁰ρ⁰ enhancement is seen is our data, in direct contradiction with the earlier, lower statistics experiment.     

Mean-square atomic displacements in f.c.c. crystals

The mean-square displacements of volume atoms, surface atoms and atoms near a vacancy are calculated for f.c.c. metals and rare gas solids, using a simple series expansion of the Wallis formula. We employ a model with rotationally invariant bond bending forces, unlike the de Launay model used by Masri and Dobrzynski. For atoms near a vacancy and for rare gas solids this method has been applied for the first time. The calculations are done to the R⁴ term in the expansion. The inclusion of the R⁴ term together with temperature-dependent elastic constants give results which are in better agreement with experiment. For rare gas solids Lindemann ratios calculated by this method agree closely with the values obtained from elaborate computations as well as those deduced from entropy data. The present calculation also supports the view that the melting of solids is initiated at the crystal surfaces and in the neighbourhood of vacancies.     

Relating c < 0 and c > 0 conformal field theories

A ‘canonical mapping’ is established between the c = −1 system of bosonic ghosts and the c = 2 complex scalar theory and a similar mapping between the c = −2 system of fermionic ghosts and the c = 1 Dirac theory. The existence of this mapping is suggested by the identity of the characters of the respective theories. The respective c < 0 and c > 0 theories share the same space of states, whereas the spaces of conformal fields are different. Upon this mapping from their c < 0 counterparts, the (c > 0) complex scalar and the Dirac theories inherit hidden non-local sl(2) symmetries.     

Determination of the total c\bar c production cross section in 340 GeV/c \Sigma^--nucleus interactions

The production of charmed particles by 340 GeV/c momentum was studied in the hyperon beam experiment WA89 at the CERN-SPS, using the -spectrometer. In two data-taking periods in 1993 and 1994 an integrated luminosity of on copper and carbon targets was recorded. From the reconstruction of charm particle decays in 10 decay channels production cross sections for D, , and were determined in the region . Assuming an dependence of the cross section on the nucleon number, we calculate a total production cross section of per nucleon. The last term is an upper limit on the unknown contribution from charmed-strange baryon production. Received: 3 September 1999 / Published online: 3 February 2000     

Global ac susceptibility of low pinning high-T c crystals near T c

We have studied the hysteretic loss and the shielding capability of Bi₂Sr₂CaCu₂O₈and YBa₂Cu₄O₈ crystals as a function of ac field (μ ₀ H ≤ 1mT), frequency, and temperature at small dc fields (μ₀ H _a < 1 mT) near the critical temperature T _c . We show that the global ac field dependence of the susceptibility can be described in terms of a geometrical barrier model in agreement with local permeability results. At temperatures very near T _c , however, we observe a crossover to diffusive behavior. The frequency dependence of the temperature of the dissipation maximum at finite dc fields shows a thermally activated behavior.     

Interatomic forces in the b.c.c. transition metals

Using information obtained from X-ray scattering data, a model for the charge density in the transition metals is constructed. In this model the charge density is given by the sum of the charges situated at the ionic and bond centre sites. Within this framework, expressions for the second order elastic constants and the phonon frequencies in the cubic transition metals are obtained. These quantities have been calculated with and without the bond charge contributions for iron. The comparison with the experimental results shows that the introduction of bond charges, and hence angularity in the force-fields, in iron leads to distinctly better agreement with experiment. From a study of the elastic constants of the other b.c.c. transition metals some observations as to the nature of the angular force fields are made.     

Sputtering induced topography development on f.c.c. metals

The generation and development of surface features by ion bombardment induced sputtering erosion of f.c.c. metals is reviewed. Studies with polycrystalline substrates reveal the plethora of individual features, both etch pits and cones and the repetitive features, such as ripple trains, which form differently on different crystallite surfaces. Studies with well defined single crystals clarify the origins of such features and the relative independence of their habit on ion species and substrate material.     

Electronically induced lattice instabilities in b.c.c. Zr

Electronically induced lattice instabilities in metals are discussed by means of an accurate determination of the generalized susceptibility function, χ(q), from a KKR band structure and Fermi surface determined for b.c.c. Zr. We describe how the detailed behavior of χ(q) for b.c.c. Zr may be related to the omega phase upon alloying with Nb or upon application of pressure to the b.c.c. lattice.     

Hall effect studies in ytterbium through the f.c.c.-h.c.p. transformation

The Hall coefficient (R_H) of polycrystalline Yb showing the f.c.c.-h.c.p. transformation has been determined at 20 koe in the range 1.8–420°K. Both the structural and the associated magnetic transformation would separately be expected to modify R_H, but the evidence suggests that the observed behaviour through the transition arises primarily from the effects of the change in structure. A more detailed interpretation is presently not possible, however, because of the lack of knowledge of the band structure of h.c.p. Yb.